This invention relates to a gantry-type jig by means of which tools or workpieces can be moved and/or positioned with a high degree of accuracy along very accurate, programmed paths within the space covered by the gantry-type jig. For this purpose, a trolley is provided to carry the tool or the workpiece, with the trolley travelling on a cross-rail. The cross-rail comprises a load-bearing element and a positioning element. The load-bearing element takes up the full, or at least the major part of the load, whereas the positioning element is subjected to no load, or at most to only a minor load, and governs the accuracy in the vertical direction.
Gantry-type jigs fulfilling requirements of this kind are commonly known, for instance for measuring purposes. In order to obtain the required degree of accuracy, the stationary frame parts as well as the traveling cross-rail are of such sturdy construction that deflection resulting from the traveling load is far smaller than is required by the tolerances of accuracy. Therefore, the construction has to be relatively heavy and involves great material expenditure. The use of light-weight materials having a high elastic modulus, for instance chemical-fiber reinforced plastics, are mostly generally not used due to the high prices of these materials.
A measuring apparatus in the form of a jig is already known from West German Offenlegungsschrift No. 27 44 687, which discloses a jig traveling on a platform. The cross-rail of the jig comprises a positioning element guiding the trolley and a load-bearing element supporting the weight of the trolley. The degree of accuracy of the measuring apparatus already known depends on how uniformly the weight of the trolley is supported on the load-bearing element. In this respect it has to be taken into account that the trolley is supported via a spring and that the spring is automatically less tensioned with the trolley in a central position due to the largest deflection of the load-bearing element that occurs in this case than with the trolley in the vicinity of the longitudinal beams of the frame. Hence, the positioning element is subjected to differing bending forces, resulting in inaccuracies. Apart from this, the measuring apparatus according to German OS No. 27 44 687 is not of the gantry-type.